Your optimum is what your build can handle. Different powersets and builds will have varying results.
I think I have my AR/EM at +1 level and considered as 3-4 heroes. I could probably dial it up a little more, but it's a decent challenge against most mobs (and some nasty groups like Malta can make it a real challenge, depending on how the hit rolls go against my defense) that is still fun. I like to balance between the two, as too easy isn't fun, but neither is constant, high difficulty.
I think that works well for me as it gives me a fair amount of mobs, but they aren't too high level. I'd probably lower how many heroes I was considered if I raised their level, but it wouldn't be as fun to have fewer targets, heh. I tend to fiddle with it until it feels right for the Blaster or character I am on.
Guide: Tanking, Wall of Fire Style (Updated for I19!), and the Four Rules of Tanking
Story Arc: Belated Justice, #88003
Synopsis: Explore the fine line between justice and vengeance as you help a hero of Talos Island bring his friend's murderer to justice.
Grey Pilgrim: Fire/Fire Tanker (50), Victory
My single-target oriented blapper usually just pumps levels. 1 player, +2 levels. While he can handle more enemies, there's really no reason to have them around when he's only killing one at a time.
My fire/fire AoE blaster says "lol cimerora ranged attacks" and pumps players. 8 players, +0 levels; it's lawnmower time!
The Bacon Compels You.
My Dp/Fire is level 45 atm and i run a farm set to 0x6. I'll prolly bump it to 0x8 and try it because i'm not dieing atm. I need to see whats the most i can handle. I don't wanna go up +1 or 2 because i know i'd faceplant. Id rather just kill bigger mobs. lol
Depends on the enemy, if its something that doesnt mez and doesn't have high lethal/psi resistance, I'll run it +2/8 without bosses. But that's on a level 50 IO'd out arch/mm, +0/6 or +1/6 might be more suited for an aoe blaster leveling up.
Pre-SOs: +0/x1 on pretty much every toon, just to maximize mission speed.
Once I get some decent enhancements I generally boost the level by one or two since I mostly play ST-heavy Blasters. I'm thinking of bumping my Archery / Energy up to +2/x4 or maybe +1/x6 (he's at +2/x1 now) but really I rarely solo on him so I've just never bothered cranking the difficulty more. 
(Pure ranged archers are a blast on TFs). And if I ever finish my Ice/Elec I may try +4/x1/Bosses just to see if I can do it...
Cascade, level 50 Blaster (NRG/NRG since before it was cool)
Mechmeister, level 50 Bots / Traps MM
FAR too many non-50 alts to name
[u]Arcs[u]
The Scavenger Hunt: 187076
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AR/Dev, I go -1/x3 with Bosses. I could probably up the difficulty or add another notch to team size but my current settings allow me to pretty casually mow through missions.
It definitely depends on what you're comfortable with and what build you're playing, IMO.
My lvl 50 E3 solos on non-Cimeroran +3/+3 spawns with Bosses and no AV's solo without a problem (it was my first toon, and I still find it very easy to solo with). I can finally do +2/+3 on my lvl 50 F3, but its still *very* dicey, and RotP figures into the decision to solo on those settings at some point (which I'm not exactly proud to admit, but hey... what good is pride anyway?).
My lvl 50 Psy/MMM/EM and my lvl 32 Ice/Ice are fine on +2/+3. Things occasionally get interesting, but usually +2/+3 is very manageable for them. Someone who plays more often than I have been able to over the past year+ might be able to do more.
Sticking to about +0/x5 no bosses on my AR/En. AoE and ST beast, but I don't trust my connection and computer speed enough to handle more difficult spawns.
I am the Blaster, I have filled the role of Tank, Controller and Defender
Sometimes all at once.
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What settings do you have for your Blaster when Solo, & at what levels do you change them?
How many Standard Heros do you equal?
What level do you set your opponents to?
Do you set all boss down to EB?
Just started a DP/Devices Blaster & wanted some advice as what is best for optium XP.